1124 
DE. J. S, BOWEEBANK ON THE ANATOMY 
beautiful spicula of the ovaria vary in form and size, in passing from one species to 
another, in a perfectly unmistakeable manner, so that, if the organs of reproduction be 
present, which is most frequently the case, the species may be readily recognized from 
their spicula only. But in other cases, and even in the same genus in the absence of the 
ovaria, the differences between two nearly allied species are equally well determined by 
the spicula of the dermal and interstitial membranes. Thus, in our two species of 
British Spongilla^ S.flimatilis has no tension spicula different from those of the skeleton, 
while in 8. lacustris we find the fusiformi-acerate entirely-spined spiculum, represented 
in fig. 21, Plate XXIV. Phil. Trans. 1858, in abundance. So likewise in two species 
of Tethea, T. cranium from Shetland, and T. simillima, Bowerbank, MS., from the 
Antarctic regions, the only well-determined difference that exists is, that the sarcode of 
the former is profusely furnished with exceedingly minute sigmoid spicula, while that 
of the latter is enthely destitute of them. It will therefore be seen that these exceed- 
ingly minute organs frequently afford the most valuable and certain means of discrimi- 
nating species. But although so minute, we must not imagine that it is very difficult 
to obtain these characteristic evidences ; for, as I shall show more at length hereafter, it 
requires but the dissolution of a small piece of the sponge in hot nitric acid to at once 
furnish us with a general view of the whole of the spicular contents of the sponge under 
examination ; so that, to one who has become familiarized with the general character- 
istics of the forms and sizes of the different classes of spicula peculiar to each organ of 
the sponge, such a preliminary observation at once indicates the nature and especial 
seat of the principal specific characters of the subject under examination. 
In some sponges the relative variation in size of the adult skeleton-spicula is greater 
than in others ; but this variation, although sometimes a substantial character, must 
not be always assumed to be correct, as in young sponges with simple forms of skele- 
ton it is very difficult to discriminate between the young and only partially developed 
spicula and the adult ones. Thus in a young specimen of Spongilla Jluviatilis, I found 
in the same field of view one spiculum perfectly well proportioned which measured 
3 -|^th of an inch in length and yo of an inch in diameter, another -girst of an 
inch in length and y^o^th of an inch in diameter, — the length and diameter of an 
average-sized spiculum of the species in a fully developed condition being, length -^th 
of an inch, and diameter inch. 
Abnormal or immature forms must not be mistaken for fully developed and normal 
ones, as we find in some of the more complicated forms of spicula that the development 
of form is quite as progressive as that of size, as instanced in figs. 4, 5, 6 & 7, Plate 
XXIV. Phil. Trans. 1858, which represent the progressive stages of development of 
the spinulo-recurvo-quaternate form of spiculum, and also in figs. 4, 5, 6, 7 & 8, Plate 
XXV. Phil. Trans. 1858 — where the first four figures represent the progressive deve- 
lopment of the dentato-palmate inequianchorate spiculum, and the last an abnormal 
form, probably arising from arrested development. 
